1. Field of the Invention
The present invention deals with a system and method for providing both engine and regenerative braking in a hybrid electric vehicle.
2. Disclosure Information
Hybrid electric vehicles have been under development by various automotive companies for several years. Such vehicles typically utilize both an internal combustion engine and a rotating electrical machine such as a traction motor/generator for propulsive power. The rotating electrical machine is typically coupled to a storage battery which is recharged during the course of regenerative braking.
If the rotating machine incorporated in a hybrid motor vehicle, as well as the associated conductors and energy storage device (battery) have sufficient capacity, it is possible by means of regenerative braking to convert a good deal of the kinetic energy of the moving vehicle into stored charge within the vehicle""s traction battery. If however, as is frequently the case, either the rotating electrical machine and associated battery and conductors are not capable of handling the high current flux which would be involved in conversion of kinetic energy, or other system capacity issues exist, it is necessary and desirable to use the combination of both regenerative braking and engine braking to achieve desired dynamic braking of the vehicle.
In order to promote the acceptability of hybrid electric vehicles to the motoring public, it is desirable to implement a braking system wherein the vehicle""s driver is not aware that regenerative braking is occurring. Thus, the driver will not perceive a variable dynamic braking characteristic which would otherwise occur if maximum engine braking were augmented by regenerative braking. In other words, it is desirable that the dynamic braking of the vehicle that is, the braking due solely to engine brake torque and regenerative braking, if any, closely approximate the braking xe2x80x9cfeelxe2x80x9d which is present when the engine alone is used for dynamic braking.
A system and method according to the present invention achieves the desired dynamic braking experience by reducing the brake torque produced by the engine in the event that regenerative braking is employed. U.S. Pat. No. 6,026,921 discloses a braking system for a hybrid vehicle having an internal combustion engine and a motor/generator. The ""921 patent does not however, teach or suggest a system in which an engine brake torque is decreased at the same time regenerative brake torque is increased.
As used herein, the term xe2x80x9cbrake torquexe2x80x9d refers to the dynamic braking capability provided by either an engine, or by a regenerative braking system. Stated another way, as the term xe2x80x9cbrake torque of the enginexe2x80x9d, means herein that the engine is capable of slowing the vehicle by providing a negative torque output having a variable magnitude determined by various engine operating parameters.
A method for braking a hybrid electric vehicle having an internal combustion engine and a traction motor coupled to a common output shaft, with the vehicle further having an electric energy storage device connected to the traction motor, includes the steps of monitoring operation of the vehicle to determine if the vehicle""s driver is operating the vehicle in a braking mode, and in the event that the vehicle is being braked, controlling the engine and the traction motor so as to provide dynamic braking such that when the traction motor is operated regeneratively, the brake torque produced by the engine is reduced from a maximum contemporaneous brake torque value. The sum of the brake torque produced by the engine and the brake torque produced by the traction motor when the vehicle is braking is equivalent to the maximum contemporaneous brake torque from the engine. As a result the vehicle""s driver will not perceive a brake torque spike caused by brake torque in excess of that torque which would normally be produced by the engine alone.
According to another aspect of the present invention, the present method may include the steps of monitoring the state of charge of the electric energy storage device and controlling the engine and traction motor so as to provide dynamic braking solely from the engine in the event that the state of charge is greater than a predetermined charge threshold.
If a hybrid electric vehicle according to the present invention is equipped with a torque converter, the torque converter will be locked by a controller when dynamic braking is occurs.
According to another aspect of the present invention, a braking system includes a controller for receiving a state of charge signal, a braking signal, and signals from other onboard monitors. In the event that the vehicle is being braked, the engine and traction motor will be controlled so as to provide dynamic braking such that when the state of charge of the electric energy storage device is less than a predetermined charge threshold, the traction motor is operated regeneratively and the brake torque produced by the engine is reduced from a maximum contemporaneous brake torque to a lesser brake torque value.
The brake torque produced by the engine may be controlled through a plurality of methods known to those skilled in the art and suggested by this disclosure. Such methods include the opening of the engine throttle to by means of an electronic throttle control or the disabling of the intake and exhaust valves of the engine""s cylinders, or by adjusting of a cylinder valve camshaft drive mechanism. In any event, these three methods all reduce the torque output of the engine or more precisely, the ability of the engine to absorb power, so as to avoid the problem of the vehicle operator perceiving the dynamic braking as changing in character.
As noted above, it is an advantage of the system and method of the present invention that dynamic braking will be seamlessly perceived by the driver as unchanging, regardless of whether the traction motor battery is charged or uncharged because in the event regenerative braking is used, the engine braking capability will be diminished or decreased such that the overall dynamic braking will remain the same.
Yet another advantage of the present invention resides in the fact that a system and method according to this invention is well suited for the provision of regenerative braking with a hybrid electric vehicle in which the rotating electrical machine and the engine are mounted to a common output shaft, such as an engine crankshaft, and with the rotating electrical machine and the engine being locked together and rotating at precisely the same speed at all times.
Other advantages, as well as objects and features of the present invention, will be become apparent to the reader of this specification.